1. Field of the Invention
The present general inventive concept relates to an earphone, and more particularly, to an open-air type earphone having a duct that communicates between an inner portion of the earphone and an outer portion of the earphone and applies an inductance component to an acoustic signal generated by an electroacoustic transducer.
2. Description of the Related Art
Earphones are tiny speakers that fit into a user's ears and have an electroacoustic transducer that converts an electric signal into an acoustic signal.
Earphones can be classified as a closed-air type earphone and an open-air type earphone according to the shape of a housing in which an electroacoustic transducer is contained. Closed-air type earphones are configured such that a housing is hermetically closed from the surrounding atmosphere, and open-air type earphones are configured such that small back holes are formed along an edge of a rear portion of a housing to communicate between the inside and the outside of the housing.
In closed-air type earphones, since the sound pressure in the ear changes according to how tight the earphone fits into the ear, the sound quality can also vary. However, in the open-air type earphones, since the inside and the outside of a housing communicate with each other, the sound pressure inside the ear can be maintained constant over a wide range of frequencies from a low frequency to a high frequency. Additionally, acoustic resistance materials, e.g., urethane foams, may be embedded in back holes formed in the housing of the open-air type earphones to reduce external noise.
Resonance in the open-air type earphone occurs at a frequency between a middle frequency and a high frequency of an acoustic signal according to the size of the back holes. This resonance results in a sound pressure peak between the middle frequency and the high frequency, thereby degrading frequency characteristics of the open-air type earphones. In an effort to address these problems, U.S. Pat. No. 4,742,887 describes an open-air type earphone having a duct.
FIG. 1 is a cross-sectional view illustrating a conventional open-air type earphone.
Referring to FIG. 1, the conventional open-air type earphone includes an electroacoustic transducer 102 including a permanent magnet, a voice coil, and a diaphragm, and a housing 104 accommodating the electroacoustic transducer 102. Back holes 106 are formed in the back of the housing 104 and are covered by acoustic resistance materials such as non-woven fabrics. A duct 108 extends from a side of the housing 104.
In the conventional open-air type earphone having the back holes 106, since the frequency response decreases at frequencies below the resonant frequency of the vibration system consisting of the voice coil and the diaphragm, the resonant frequency of the electroacoustic transducer 102 should be as small as possible in order to improve the low frequency characteristic.
The resonant frequency of the electroacoustic transducer 102 may be decreased by increasing the compliance or the equivalent mass of the electroacoustic transducer 102. Here, the compliance is a measure of the flexibility of a moving body. For example, a high compliance speaker is very soft at a cone support portion.
In particular, in order to increase the compliance of the electroacoustic transducer 102, it is necessary to either (1) select a material of high compliance for the diaphragm or (2) decrease the thickness of the diaphragm. However, there are limits regarding the compliance of the material that can be used for the diaphragm and the extent to which the thickness of the diaphragm can be reduced. Further, by increasing the equivalent mass of the electroacoustic transducer 102, the sensitivity and acoustic characteristic of the earphone in the high frequency range is deteriorated.
In the conventional open-air type earphone of FIG. 1, the compliance and the equivalent mass of the electroacoustic transducer 102 are improved by extending a portion of the housing 104 to form the duct 108. Since the duct 108 adds an equivalent mass to the vibration system, the resonant frequency of the vibration system is reduced by the amount corresponding to the added equivalent mass. That is, this reduction of the resonant frequency of the vibration system is achieved irrespective of the compliance and the equivalent mass of the vibration system. Accordingly, the low frequency characteristic of the conventional open-air type earphone can be improved due to the duct 108.
The low frequency characteristic of the earphone is basically determined by the equivalent mass of the duct 108 and the resonant frequency of the vibration system, but also is determined by how tight the earphone fits in the ear. That is, the low frequency characteristic is changed according to the leakage of sound when an acoustic signal generated by the earphone is transmitted to the ear. The low frequency component of the acoustic signal is reduced when there is a great deal of sound leakage.
Additionally, since the hearing sensitivity of different users varies based on ear structure, the low frequency characteristic of the earphone is also affected by the ear structure as well as the equivalent mass of the duct 108 and the resonant frequency of the vibration system.
Users may also want to adjust the low frequency characteristic of the earphone according to the music genre. Here, the low frequency ranges from 20 to 200 Hz, and can be divided into deep bass ranging from 20 to 40 Hz, middle bass ranging from 40 to 400 Hz, and upper bass ranging from 100 to 200 Hz. For example, deep bass is particularly important when listening to classical music, whereas upper bass is particularly important when listening to hip-hop or dance music.
Accordingly, the low frequency characteristic should be adjusted according to a user's physical feature (i.e., the ear structure), taste, and music genre.